1. Field of the Invention
The present invention relates to a one cycle control power factor correction (PFC) control circuit with dynamic gain control. More specifically, the present application relates to a one cycle control PFC control circuit for a switching converter in which a gain of a current sense amplifier is varied based on the input line voltage.
2. Related Art
Power factor correction control in switching converters typically involves modulating the duty cycle of the switching element in the converter such that the input appears to be purely resistive. For those control circuits that use a one cycle control technique, for example, in controlling a boost converter, the output of the voltage error amplifier in the converter control loop, that is, the error voltage VCOMP, is integrated over the switching cycle to produce a ramp voltage. The ramp signal is then typically compared to a reference voltage which is typically generated by a combination of inductor sense current voltage and VCOMP to determine the duty cycle of the boost converter power switch. One non-limiting example of such a control circuit is Assignee International Rectifier Corporation's IR1150 uPFC One Cycle Control PFC Integrated Circuit.
FIG. 1A is a block diagram of the IR1150. FIG. 1B is a schematic of an application circuit in which the IR1150 is suitable for use. The IR1150 is preferably used to control the duty cycle of the switch Q1 of the boost converter illustrated in FIG. 1B. Specifically, the switch Q1 is controlled to convert an input voltage VIN, typically provided from an AC line voltage via a rectifier bridge (BRIDGE), as illustrated in FIG. 1B, into a desired output voltage VOUT. Specifically, the IR1150 controls the gate of the switch Q1 via a control signal provided at the output GATE pin (pin 8). The control signal turns the switch Q1 ON and OFF to provide the desired output voltage VOUT.
While the operation of the IR 1150 is well known, a brief review of its features is useful. The IR 1150 includes a COM pin (pin 1) that provides a connection to ground and a supply pin VCC (pin 7) which is preferably connected to a supply voltage VCC to supply power to the IC. The feedback pin VFB (pin 6) is an input which provides a signal indicative of the output voltage VOUT. Preferably, this signal is supplied via the voltage divider formed by the feedback resistors RFB1, RFB2, RFB3. The compensation pin COMP (pin 5) is connected to external circuitry (Rgm, Cz, Cp) that compensates the internal voltage loop and soft start time. This pin is also connected to the output of the voltage error amplifier 20 (see FIG. 1A). The current sense input ISNS (pin 3) is the inverting current sense input and peak current limit. The voltage provided at this pin is the negative voltage drop, sensed across the system current sense resistor Rs which represents the inductor current through the inductor L1. The over voltage protection pin OVP (pin 4) is connected to an input of the over voltage protection comparator 30 which prevents an over voltage condition. More specifically, the over voltage protection pin OVP is provided with a signal indicative of the output voltage, preferably via the voltage divider provided by the resistors ROV1, ROV2, ROV3 in FIG. 1B, for example. If the output voltage exceeds a threshold level, the IR1150 preferably enters a fault mode.
One problem that arises from the one cycle control technique mentioned above and used in the IR1150 is that the system cannot provide overpower protection when the line voltage is any higher than the minimum permissible line voltage that the system is designed for.
Accordingly, it would be desirable to provide a control circuit that avoids these problems.